Pressure vessels are commonly used for containing a variety of fluids under pressure, such as hydrogen, oxygen, natural gas, nitrogen, propane, methane and other fuels, for example. Generally, pressure vessels can be of any size or configuration. The vessels can be heavy or light, single-use (e.g., disposable), reusable, subjected to high pressures (greater than 50 psi, for example), low pressures (less than 50 psi, for example), or used for storing fluids at elevated or cryogenic temperatures, for example.
Suitable pressure vessel shell materials include metals, such as steel; or composites, which may include laminated layers of wound fiberglass filaments or other synthetic filaments bonded together by a thermal-setting or thermoplastic resin. The fiber may be fiberglass, aramid, carbon, graphite, or any other generally known fibrous reinforcing material. The resin material used may be epoxy, polyester, vinyl ester, thermoplastic, or any other suitable resinous material capable of providing fiber-to-fiber bonding, fiber layer-to-layer bonding, and the fragmentation resistance required for the particular application in which the vessel is to be used. The composite construction of the vessels provides numerous advantages, including lightness in weight and resistance to corrosion, fatigue and catastrophic failure. These attributes are due at least in part to the high specific strengths of the reinforcing fibers or filaments.
A polymeric or other non-metallic resilient liner or bladder is often disposed within a composite shell to seal the vessel and prevent internal fluids from contacting the composite material. The liner can be manufactured by compression molding, blow molding, injection molding, or any other generally known technique. Alternatively, the liner can be made of other materials, including steel, aluminum, nickel, titanium, platinum, gold, silver, stainless steel, and any alloys thereof. Such materials can be generally characterized as having a high modulus of elasticity. In one embodiment, the liner 20 is formed of blow molded high density polyethylene (HDPE).
FIG. 1 illustrates an elongated pressure vessel 10, such as that disclosed in U.S. Pat. No. 5,476,189, entitled “Pressure vessel with damage mitigating system,” which is hereby incorporated by reference. Vessel 10 has a main body section 12 and substantially hemispherical or dome-shaped end sections 14. A boss 16, typically constructed of aluminum, is provided at one or both ends of the vessel 10 to provide a port for communicating with the interior of the vessel 10. As shown in FIG. 2, vessel 10 is formed with an inner polymer liner 20 covered by an outer composite shell 18. The composite shell 18 resolves structural loads on the vessel 10.
FIG. 2 illustrates a partial cross-sectional view, taken along line 2-2 of FIG. 1, of a typical end section 14 including boss 16, such as that disclosed in U.S. Pat. No. 5,429,845, entitled “Boss for a filament wound pressure vessel,” which is hereby incorporated by reference. The boss 16 typically has a neck 22, a port 26 allowing fluid communication with the interior of vessel 10, and an annular flange 24 extending radially from port 26. Boss 16 is fit to outer shell 18 and liner 20 such that port 26 extends between the interior and exterior of pressure vessel 10. Typically, shell 18 abuts neck 22. Generally, flange 24 is contained between portions of liner 20 and/or is sandwiched between the liner 20 and the shell 18. In certain embodiments, flange 24 may include at least one annular groove 32 shaped to accept corresponding annular tab(s) 34 on liner 20. This construction secures the boss 16 to the vessel 10 and provides a seal at the interfaces between the boss 16, shell 18, and liner 20.
A method of forming a pressure vessel 10 includes mounting a boss on a mandrel and allowing a fluid polymer material for liner 20 to flow around flange 24 and into groove 32 of boss 16. The liner material then solidifies, thereby forming a portions of liner 20 adjacent to flange 24 and tab 34 received within groove 32. Liner 20 is thereby mechanically interlocked with boss 16. Accordingly, even under extreme pressure conditions, separation of liner 20 from boss 16 is prevented.
In an exemplary embodiment, outer shell 18 is formed from wound fibers and surrounds the liner 20 and at least a portion of flange 24 of boss 16. In an exemplary method, a dispensing head for the fibers moves in such a way as to wrap the fiber on the liner 20 in a desired pattern. If the vessel 10 is cylindrical, rather than spherical, fiber winding is normally applied in both a substantially longitudinal (helical) and circumferential (hoop) wrap pattern. This winding process is defined by a number of factors, such as resin content, fiber configuration, winding tension, and the pattern of the wrap in relation to the axis of the liner 20. Details relevant to the formation of an exemplary pressure vessel are disclosed in U.S. Pat. No. 4,838,971, entitled “Filament Winding Process and Apparatus,” which is incorporated herein by reference.
Although the liner 20 provides a gas barrier under typical operating conditions, the design of a pressure vessel 10 of this type produces a phenomenon wherein gas diffuses into the liner 20 under pressurization of vessel 10. When depressurization of the vessel 10 occurs, this gas diffuses out of the liner 20, and in some cases into the space between the liner 20 and the shell 18. A pocket of gas may be formed, causing the liner 20 to bulge slightly inward and possibly become stretched. Moreover, gas at the interface between the liner 20 and the shell 18 can promote undesirable separation between the liner 20 and shell 18. Additionally, upon re-pressurization, the gas trapped between liner 20 and shell 18 may be expelled abruptly through microcracks in shell 18 that form at high pressures. The relatively sudden expulsion of gas can set off leak detectors, when, in actuality, pressure vessel 10 exhibits no steady leak.